The rapid advances made in epitaxial growth on GaAs-type substrates has led to the development of a new class of electromagnetic wave detectors using the absorption of radiation around a wavelength λ corresponding to the electron transition between various energy levels within one and the same band or between the valence band and the conduction band. The diagram shown in FIG. 1 illustrates this type of transition.
The recent evolution in performance of this type of component is due in particular to the relatively easy production of semiconductor heterojunction multilayers in the standard system by MBE (molecular beam epitaxy), that is to say the GaAs/Ga(1-x)AlxAs. By adjusting the growth parameters, the thickness of the quantum wells and the percentage x of aluminum in the barriers imposing the confinement potential, it is possible to choose a narrow detection band (about 1 micron) centered on a given wavelength.
This type of structure has the advantage of providing very good sensitivity because of the discretization of the energy levels within the conduction bands of the photoconductor materials used.
Within the context of intersubband transitions, so that this type of transition is possible, it is necessary for the electric field of the incident electromagnetic wave to have a component along the direction of growth of the layers, i.e. along the direction D indicated in FIG. 1, this direction being perpendicular to the plane of the layers. The consequence of this physical effect is that a detector has little or no absorption in the case of illumination at normal incidence.
It has already been proposed to use coupling means of the diffraction grating type (cf. Goossen and Lyon, APL 47 (1985), pp 1257-1259) for generating said perpendicular component, creating diffracted rays, especially lamellar (1D) gratings or steps for coupling only a single polarization of the light. However, crossed diffraction gratings are also known for coupling the various electric field components of incident radiation, as illustrated in FIG. 2. The matrix grating Rij diffracts the incident radiation along both the direction Dx and the direction Dy. The major drawback of this type of matrix structure lies in the depth d for diffractive features that are generally produced within an encapsulation layer EL, thereby protecting the multiple quantum well MQW structure as illustrated in FIG. 3.